This invention pertains to plastic injection molding equipment, and more particularly to an apparatus for separating the runner from the plastic part during the operation of the molding dies.
During the injection molding of thermoformed plastic parts, some means is always used to convey the plastic from the source of molten plastic, that is the machine nozzle, to the cavity for forming the finished part. This takes place within the die. Although some systems have been developed which completely eliminate the need for a runner channel, many dies use such a channel formed in the face of the die for conveying the plastic to the cavity.
Generally speaking, die design is simpler with a runner system. However, the runner, which is the residual plastic remaining in the machined runner channels, is ejected along with the part and must be separated therefrom for the operation to be complete. This often involves a secondary trimming operation.
Two systems have previously been developed for automatically removing the runner during the ejection, or opening cycle, of the molding dies. In one case, the die is built with a third, movable plate. The plate is machined with the runner on one side and the part on the other. Thus, the die opens and the runner is ejected, breaking it from the part, whereupon the other side of the die opens and the part is ejected. Though effective, this system has the disadvantage of being a costly die to build and is noisy in operation.
The second previously known die design which separates the runner from the part during ejection is, however, a simple two plate die. In this case, the runner is modified so that the area immediately adjacent to the cavity, known as the gate, does not enter between the two die plates, but is angled in such a way that it enters through a drilling in the side wall of the part. This is known as a sub-gate. Since the runner end which enters the cavity is encircled by the cavity wall even when the die is open, that area of the runner is broken when the part and runner are ejected. While this system affords many advantages, it has one significant limitation, however. It cannot be used on a thin part nor one in which the length-to-height ratio is too great. This is because the mold base is made of a material which cannot support the repeated stress if the sub-gate is too close to the surface. Further, the sub-gate must necessarily be small to be the weakest area to break on ejection. Thus, the part cannot be packed with mold pressure since the gate area hardens quickly.
Accordingly, it is the general object of the present invention to provide a runner shearing assembly which automatically shears the runner from the part when the parts are ejected from the mold.
Another object of the present invention is to provide such a shearing assembly made of few, simple parts which are easy to install.
Yet another object is to provide a shearing assembly which can work with a very thin part.
Still another object is to provide a shearing assembly which can shear a much larger gate.
A further object is to provide a shearing system which can be retrofit into existing dies, including aluminum and soft steel molds.
A still further object is to provide a shearing assembly which can quickly be replaced if damaged.
These and other objects and advantages, and the manner in which they are achieved will be made apparent as the specification and claims proceed.